The power factor of an a.c. power system is of economic importance because of the high cost of reactive kva. Low power factor of a system results in unnecessary distribution losses, difficult voltage regulation and oversized equipment. To correct for low power factor it is well known in the a.c. induction motor art to shunt the induction motor by a capacitor or other source of capacitive reactance. Depending upon the application, correction may be provided for a given load by means of a capacitor rated for that load. Correction may also be provided for a variety of operational conditions by utilizing a bank of graduated static capacitors in parallel with the motor or by employing an over-excited synchronous motor as the source of capacitance. As a means of compensating for lagging currents within the motor as well as in the external circuit and to avoid the effects of harmonics, Steinmetz, U.S. Pat. No. 602,920, suggested connecting the source of capacitance to the motor through a step-up transformer. The fact that a single large capacitor could be utilized to reduce costs was recognized by Weichsel, U.S. Pat. No. 1,712,237. These prior art systems have a common fault, in that they frequently overcompensate or undercompensate for the reactive kva under changing load conditions.
Incorrect compensation results in excessive system kva loading, overheating, and inefficiencies. Furthermore, overcompensation can lead to inductive and synchronous machines becoming self-excited when sufficiently large capacitive currents are present in their stator circuits. These currents can result in serious overvoltage and/or excessive transient torques. As a consequence, shunt reactors are frequently installed at the sending end of lightly loaded distribution lines as a precautionary measure to help compensate for any excessive capacitive currents.
The ideal level of compensation for any a.c. system occurs when total units of capacitive and inductive kva are equal. In order to approach even reasonably compromised levels of desired compensation using prior art methods has required that capacitance be incrementally switched to follow changes in inductive loading. This approach does not allow for ideal compensation. Furthermore, as the capacitors are switched, hazardous voltage transients are generated which contribute to insulation breakdown and failure of other equipment connected to the power grid.
In an attempt to avoid the problems of the prior systems, Rohatyn, U.S. Pat. No. 4,554,502, has proposed a power factor correcting system in which voltage applied to a large capacitor through a step-up transformer is varied by means of a mechanically adjustable variable transformer. A control and sensing circuit is employed to drive a servo motor to adjust the variable transformer through a lead screw which carries a pair of brushes. This system avoids voltage transients due to switching, but the hazards which result from overcompensation or undercompensation remain, due to the inherently slow response of the system disclosed. The response time of the servo motor, when combined with that of the lead screw, results in the lapse of a significant interval of time between the sensing of reactive kva and completion of the voltage adjustment necessary to fully compensate for it.